The invention relates to a system and a method for an intensity-modulated proton therapy of a predetermined volume within an object.
Proton therapy is an advanced method of applying cancer radiotherapy by using proton beams. It is a superior but costlier alternative to conventional therapy, which is usually applied with photon beams. Gantries for this kind of proton therapy have been published and proposed in the international patent applications WO 2001/00276 and WO 2004/026401 and in the European patent application 04 017 266.0 respectively.
In characterizing the distinction between photon beam therapy and proton beam therapy it has to be emphasized that a photon beam penetrates through the whole patient body. The dose profile is characterized by a dose maximum at about 1 cm under the skin followed by a monotonic exponential fall-off of the dose as a function of the depth. Unlike to a photon beam, a proton beam is characterized by a well-defined range of penetration of the beam with a dose maximum at the end of the range, the so-called Bragg peak. By changing the energy of the proton beam the position of the Bragg peak in the interior of the patient body can be easily controlled.
Therefore, there exist several convincing reasons for using proton therapy instead of photon therapy. Due to the well-defined localization of Bragg peaks in depth, proton therapy can provide in almost any situation a better localization of the dose to the target volume as compared to conventional therapy with photons. With this method a better sparing of the healthy tissues surrounding the tumor can be achieved. This important issue is used in difficult clinical situations, mainly when the cancer is surrounded by sensitive anatomical structures. The higher magnetic rigidity of the proton beam requires the use of bulky equipment for the accelerator and for the beam lines, which makes on the other hand this superior therapy more expensive than conventional therapy.
Modern radiotherapy is preferentially delivered on the patient lying in supine position by applying the beam from different directions using a so-called gantry. Photon gantries span a diameter of only 2 to 3 m. A proton gantry is typically 10 m long, it comprises a proton beam line mounted on a heavy rigid support (with a total weight of more than 100 tons). The rotation of the proton gantry around the patient table spans a cylindrical volume with a radius of 2 to 6 m.
Another issue of practical interest in proton therapy is the possibility to deliver the beam by using an active dynamic beam delivery, beam scanning. The scan is performed with a small proton pencil beam (with a width of <1 cm), by applying magnetic deflections to the beam in the lateral direction and by changing dynamically the beam energy to vary the proton range. The dose is literally painted to any shape in three dimensions by touching with the Bragg peak spot (delivering variable local dose through time exposure or beam intensity changes) sequentially each point on a grid within the target (conformation of the dose to the target volume).
The scanning method must be compared with the more conventional method, which is to scatter the proton beam ahead of the patient table in order to obtain an homogeneous proton fluence in the solid angle covering the tumor site. The shaping of the dose is then done in the lateral direction by using collimators and in depth by using passive ridge filters or other active modifiers like a rotating range shifter wheel (creation of a spread out Bragg peak SOBP through a spatial or time varying amount of material placed in the beam).
With proton beam scanning one can achieve a better conformation of the dose to the target volume. One can avoid the unnecessary 100% dose applied to the healthy tissue in reason of the fixed modulation of the range of the passive scattering method (constant SOBP compared to the variable SOBP of scanning, which can be varied as a function of the lateral position of the beam). The shaping of the dose is controlled completely just by computer control. There is no need to fabricate and position in the beam individually shaped hardware (the field and patient specific devices like collimators and compensators). With scanning the beam can be applied on the patient from several beam directions in sequence, without the need for the personnel to enter the treatment room (higher patient throughput to reduce costs can be achieved).
With scanning the dose distribution can be shaped to any shape, including (intentional) non-homogeneous dose distributions (with scattering a homogeneous dose is delivered by default). This possibility is the prerequisite for the delivery of the so-called intensity modulated proton therapy (IMPT), which relies on the idea to optimize the intensities of each proton pencil beam of a whole treatment all together independently of the gantry angle (simultaneous optimization of the beam spots). The constituent dose fields applied from each beam direction don't need to be homogeneous, only the sum must.
At the time of writing, the proton gantry of the Paul Scherrer Institute at 5232 villigen PSI in Switzerland (Proton therapy facility; its first beam line is there commonly known as “Gantry 1”) is still the only proton facility in the world capable of delivering therapy with an active scanning of the proton beam and capable of providing patient treatments with IMPT plans. In the context of the expansion of the PSI facility, an improved gantry for beam scanning (“Gantry 2”) is under construction. The invention described hereinafter is therefore an addendum to previous patents related to the design of the proton gantry of PSI dedicated to beam scanning (Patent applications as mentioned above).
However, also with the new gantry various problems in dose shaping and exact dose delivery have to be solved. One of these problems is the challenge of organ motions due to various reasons. Organ motion during treatment is therefore a severe problem faced by any kind of precision radiotherapy (including dynamic therapy with photons). In case that during the delivery of the scanned beam the target volume moves, the shape and the homogeneity of the dose distribution can be significantly disturbed, up to the point that the dynamic beam delivery can not be used at all. This is actually a main criterion for the choice of the cases treated on the Gantry 1 of PSI. Due to the organ motion problem presently at PSI only non-moving tumors attached to bony structures are treated with the beam scanning method.
A significant improvement can be achieved by increasing the speed of scanning such that the target can be repeatedly scanned (target repainting, rescanning). This has been a major point of development for the new Gantry 2 and is established accordingly. The methods envisaged to cope with the organ motion problem in the presence of large-movements like in the chest, are to switch off the beam, when the target is moved away from the desired position (gated beam delivery) or to follow directly with the pencil beam the displacement of the target (tracking). The best-known example of gating is the synchronization of the beam delivery within a given phase interval of the respiration cycle measured by external means (chest wall movement, control of the amount of inspired air etc.). The disadvantage of these methods is that the information on the target motion remains an indirect indication.